Digital subscriber line (DSL) technology is a well known data communication technology that enables a high data transmission over twisted pair copper telephone lines and is widely used because present telephone equipment and infrastructure known as POTS can still be operated together with the new DSL services. DSL service and POTS service can operate at the same time because the DSL service uses a frequency range above the frequency range of the POTS service. During the years, several DSL standards, respectively recommendations, have been established differing in upstream and downstream transmission rates and in range, for example asymmetric digital subscriber line recommendations ADSL, ADSL2, ADSL2+, and very high digital subscriber line recommendations VDSL, VDSL2, to meet the increasing demands of internet users. ADSL uses for example a maximum downstream transmission rate of up to 12 Mbit/s and ADSL2+ a downstream maximum transmission rate of up to 24 Mbit/s. These DSL recommendations are referred to in this application as xDSL standards for the sake of simplicity.
xDSL standards are defined by the International Telecommunication Union, ITU, which is an agency of the United Nations responsible for information and communication technologies. The ADSL standards are described e.g. by ITU G.992.5 Annex A for ADSL over POTS, in ITU G.992.1 Annex B for ADSL over ISDN, and ITU G.992.2 for a low transmission rate version ADSL light. Also for ADSL2 and ADSL2+, several standards have been established, e.g. ITU 992.3 Annex J, ITU G992.3 Annex L and ITU G.992.5 Annex M.
FIG. 1 illustrates in a schematic diagram a communications system providing both xDSL broadband data service and narrowband POTS telephone service. At the site of a central office 1, a DSL access multiplexer (DSLAM) 2 is arranged for providing broadband data via a twisted pair copper line 3, also known as a local loop, to the customer premises equipment 4 of a user. The customer premises equipment 4 comprises a splitter (SPL) 5 for separating the xDSL service for a residential gateway 6 and the telephone service for a telephone set 7. The central office 1 may comprise correspondingly a splitter 8 for separating the xDSL service and the telephone service.
Before performing an xDSL service, the residential gateway 6 initiates a handshaking procedure with the central office 1 to establish a data transmission according to a given xDSL standard. This handshaking procedure is defined by ITU standard G.994.1, also known as G.hs, and provides a consistent way of initiating the various types of xDSL residential gateways available now and in the future. During the handshaking procedure, capability lists are exchanged between the central office 1 and the residential gateway 6, and then an xDSL transmission mode is selected in accordance with the capabilities of the residential gateway 6 and the central office 1, and what the user wants or is willing to pay for.
During the handshaking procedure, a defined set of frequency carriers is used, and specified messages are exchanged in a transaction between the central office and the customer premises equipment for providing service information. Possible messages are for example a message CLR, capabilities list ATU-R, which may be send by the residential gateway 6, a message CL, capabilities list, which may be send by the central office 1, a message MR, mode request, which may be sent to request the transmission of an MS message, and a message MS, mode select, which may be sent to request the initiation of a particular mode of operation. The information included in these messages is in particular vendor identification, service and channel parameters and available modulations and protocols. After a transaction, a common mode of operation is selected and in a cleardown procedure the G.994.1 handshaking procedure is terminated.
Each message contains up to three message information fields, which are encapsulated in a frame. A message information field consists of three components: An identification field, followed by a standard information field, and an optional non-standard information field. The identification field and the standard information field include parameters relating to particular transmission modes, features or capabilities associated with the central office and the customer premises equipment. The identification field includes also a parameter field containing parameters independent of the mode to be selected and which includes an information whether a non-standard information field is included or not. The non-standard information field includes information beyond that defined in the G.994.1 handshaking procedure. When a non-standard information field is to be sent, the non-standard field parameter is set to binary one in the identification field of the transmitted message.
During the handshaking procedure, it is the central office who is proposing a profile to the customer premises equipment and the customer premises equipment must follow. The profile consists in a set of parameters such as minimum bitrates, maximum bitrates, modes that are supported, type of connection, impulse-noise protection INP, delay setting and so on. This information is mostly contained in the CL and CLR messages. After having received these messages, an algorithm calculates what would be the best framing parameter choice in order to match the central office profile, and the handshaking procedure continues until synchronization is reached.
After the selection of an xDSL transmission standard, a training procedure follows, during which each of the frequency channels of the selected xDSL standard are tested with regard to transmission parameters, for example signal/noise ratio S/N, interference, damping and other cable characteristics of the local loop, and for each frequency channel an optimized bit rate is determined according to its transmission characteristics. Then, frequency channels having a sufficient signal/noise ratio are selected, and an appropriate modulation is chosen for the selected frequency channels. During the training procedure, the residential gateway is synchronized with the central office, and the residential gateway remains synchronized until the end of a session.
There can be cases in which, due to external factors such as noise, bad wiring or long distances, the constraints of the profile provisioned by the central office will conflict with the line condition and this will result in that the algorithm is not able to find a solution. In other words, the modem will not reach synchronization after the handshaking procedure. These are exceptional cases but in the unlikely situation that these conditions occur, there is no escape mechanism in the standard to deal with the situation with the result that the residential gateway tries endlessly to achieve service with no success. In such a case, the central office cannot offer an xDSL broadband transmission service for the customer, and since no communication to the customer premises equipment is possible, the central office can also not understand the reason why the handshaking procedure has failed.
WO 2010/022174 discloses a DSL communication system and a method for configuring a DSL communication system to provide a robust digital subscriber loop communication. The method includes training at least one anchor DSL modem transmitter to transmit information over at least one anchor line at an anchor bit rate that is determined based on an allowable tolerable noise level and a minimum allowable signal-to-noise ratio (SNR) margin; training one or more other DSL modem transmitter to transmit information over one ore more other line at a bit rate that is determined by a current noise level and a target SNR margin, wherein the target SNR margin is higher than the minimum allowable SNR margin.
US2003063600 discloses an ADSL modem that can effectively perform a re-initialization process without terminating a data communication. The ADSL modem monitors the communication status during a data communication, determines whether the current communication status needs a re-initialization and requests a re-initialization of an opposing ADSL modem apparatus by transmitting ClearEOC when a re-initialization is necessary. It is therefore possible to perform a re-initialization process during the data communication.